1. Field of the Invention
This invention resides in the field of combustion systems for liquid-fuel rocket engines, and is particularly concerned with heat exchange structures for expander cycle rocket engines.
2. Description of the Prior Art
Rocket engines such as those used on space missions require both high thrust and a durable construction that can withstand the extreme conditions of temperature and pressure encountered upon takeoff and in flight and that will enable repeated use of the engines in successive firings. Expander cycle rocket engines use regenerative cooling to achieve both high thrust and durability. In the typical expander cycle rocket engine, the combustion chamber walls and nozzle walls are cooled by uncombusted fuel fed by a turbopump into a jacket that surrounds the chamber and nozzle. The heated fuel emerging from the jacket is cycled through the turbine side of the turbopump to serve as the driving medium for the pump. The expanded fuel emerging from the turbine side of the pump then passes into the combustion chamber injector for combustion with oxidizer in the combustion chamber. The cycle thus cools the walls of the combustion chamber and nozzle while converting a portion of the heat generated by the engine to a higher flow rate of fuel to the combustion chamber and hence a higher pressure the combustion chamber, and using the rest of the heat to preheat the fuel prior to the fuel being fed to the combustion chamber injector.
The energy that the fuel acquires in the chamber wall coolant jacket is drawn from the boundary layer of hot combustion gases inside the chamber and nozzle rather than from the bulk of the combustion gas. When heat exchange occurs only at the boundary layer, the amount of energy that can be acquired by the fuel on the coolant side is limited because of the limited surface area and the fact that excessive heat flux in the chamber can result in chamber failure.
The most efficient heat exchange structures are those that provide the most intimate contact between the medium to be cooled and the coolant, i.e., the thinnest separating walls and the narrowest flow channels which provide the greatest surface area for heat transfer. One type of structure that offers both of these features is a laminated platelet stack formed by the bonding together of very thin sheets of heat-conductive metal, the sheets having been individually chemically etched prior to bonding to form very narrow, intricate flow passages. These delicate structures can become weakened at extremely high temperatures, however, causing damage such as cracks in the walls separating the passages. For this reason, platelet structures have been used only in the jacket surrounding the chamber and nozzle where they encounter only the boundary layer of the combustion gas flow. While this avoids exposing the platelet structures to destructive temperatures, it severely limits the amount of surface area across which for heat can be transferred and hence the amount of energy that can be extracted by the coolant from the combustion gas.
It has now been discovered that unusually high heat exchange efficiency by regenerative cooling in an expander cycle rocket engine can be achieved by an injector/combustion chamber/heat exchanger assembly that divides the oxidizer feed to the rocket engine into two streams, injected separately at different locations along the direction of flow of the fuel toward the main portion of the combustion chamber, each injection followed by a different mode of heat exchange. The first stream is preferably a minor stream, i.e., with a flow rate that is substantially lower than that of the second stream, and the region into which the first stream is injected serves as a preburner in which this minor portion of oxidizer is mixed with the entire flow of fuel, causing a small portion of the fuel to burn and thereby preheat the remaining fuel. The second oxidizer stream, which constitutes the remainder of the total oxidizer fed to the engine, is injected at the entry plane of the main portion of the combustion chamber where it reacts with the preheated fuel to complete the combustion. Heat exchange occurs in the preburner section between the coolant-side fuel and the bulk of the preheated fuel rather than a boundary layer at the preburner wall. Heat exchange in the main portion of the combustion chamber then occurs between the coolant-side fuel in the heat-exchange jacket and the combustion gas near the wall of the chamber. The xe2x80x9ccoolant-side fuelxe2x80x9d is the uncombusted fuel that is heated by the engine and then directed to the turbine side of the turbopump. The terms xe2x80x9cpreheated fuelxe2x80x9d and xe2x80x9cpreheated fuel mixturexe2x80x9d are used herein to refer to the fuel-rich gas mixture that enters the main portion of the combustion chamber after having been partially burned in the preburner section with the first (minor) stream of oxidizer. This definition is used despite the fact that the temperature rise of the coolant-side fuel that emerges from the heat exchange elements is also a form of preheating.
The invention thus resides in a combination of staged combustion and staged heat exchange between the combustion gas and the uncombusted fuel that serves as the coolant. The preburner combustion product is therefore a fuel-rich mixture with a relatively small heat load compared to the combustion gas that would be produced by a non-staged system, and the heat load is low enough that the first-stage combustion gas can be cooled by a platelet-type heat exchanger. Furthermore, by drawing energy from the bulk of the preburner combustion product rather than only the boundary layer, the coolant-side fuel can draw more energy and produce a higher chamber pressure by way of the turbopump than regenerative cooling systems of the prior art.
These and other features of the invention, as well as various preferred embodiments, are described in greater detail below.